With the development of technology, energy especially renewable green energy is in great demand. Batteries as energy storage and conversion device are playing an irreplaceable role. Lithium-ion batteries are leading power sources due to their high weight and volume specific energy. The development of lithium-ion battery focuses on low cost, high energy density, long cycle life and green environmental protection.
Currently, the commercialized cathode material is lithium intercalation compounds with a layer structure (lithium cobalt oxide), spinel structure (lithium manganese oxide) or olivine structure. Lithium cobalt oxide (LiCoO2) has high theoretical capacity of 275 mAh/g, along with high cost and toxicity, and its character of easy decomposition during overcharge may cause dramatically decrease of battery capacity and safety issues. Lithium manganese oxide (LiMn2O4) with theoretical capacity of 148 mAh/g, actually less than 130 mAh/g, is unstable, of which the crystal structure may deform during the charging and discharging process, resulting in low cycling efficiency; lithium iron phosphate (LiFePO4) with theoretical capacity of 172 mAh/g has poor conductivity and reducing reversible capacity. The above conventional lithium-ion battery cathode materials cannot satisfy the requirement of battery development due to their capacity and some other problems.
Theoretical capacity of elemental sulfur is 1675 mAh/g which is much higher than those of commercialized cathode materials. The battery composed of sulfur and metal lithium has theoretical specific energy of 2600 mAh/g which is the main stream in battery development. Cathode materials containing elemental sulfur, sulfur-containing inorganic sulfides, organic sulfides, organic disulfides, organic polysulfides, or carbon-sulfur have been wildly researched by now, but some problems still exist therein.
Firstly, conductive agents have to be added due to the poor conductivity of sulfur and sulphide; secondly, solubility of polysulfides as discharge products in electrolyte may affect the cyclability of battery. Thus, researches on sulfur based cathode material focus on how to improve the conductivity and cyclability of materials solve the solubility problem of polysulfides as discharge products.
Chinese patent application CN101891930A provides a sulfur composite cathode material containing carbon nanotube, sulfur is embedded in the composite. The battery has enhanced capacity. But, the battery with high cost and complex process is not suitable for industrialization due to the use of high costly carbon nanotube.